The fabrication of electronic microcircuit components requires the use of structures such as heatsinks which are capable of dissipating the heat generated by the active parts of the microcircuit or by the soldering, brazing or glass-sealing process. Moreover, those structures that are in direct contact with the active microcircuit sections must have a coefficient of thermal expansion compatible with silicon, germanium, gallium arsenide, alumina or any material used in the fabrication of the microcircuit or their enclosures. Otherwise, stresses caused by the disproportionate expansion may damage components.
The coefficient of thermal expansion of a material is defined as the ratio of the change in length per degree Celsius to the length at 25.degree. C. It is usually given as an average value over a range of temperatures. The thermal conductivity of a material is defined as the time rate of heat transfer through unit thickness, across unit area, for a unit difference in temperature or K=WL/AT where W=watts, L=thickness in meters, a=area in square meters, and T=temperature difference in .degree. K or .degree. C.
Materials such as copper, silver, gold and aluminum which exhibit high coefficients of heat dissipation tend also to have coefficients of expansion much higher than typical microcircuit materials. For copper, K is equal to 398 W/m.degree. K whereas its coefficient of thermal expansion is about 16.times.10.sup.-6 /.degree. C. For silver, K is equal to 427 W/m.degree. K and its coefficient of thermal expansion is about 19.times.10.sup.-6 /.degree. C. By contrast, material such as gallium arsenide and silicon have an average coefficient of thermal expansion of less than 7.times.10.sup.-6 /.degree. C.
As disclosed in U.S. Pat. No. 4,680,618 Kuroda et al., it has been found convenient to use composites of copper and other low CTE metals such as tungsten or molybdenum in the fabrication of heatsinks and other heat-dissipating elements of microcircuits. Tungsten and molybdenum with average coefficient of thermal expansion of 4.5.times.10.sup.-6 /.degree. C. and 4.8.times.10.sup.-6 /.degree. C. and thermal conductivity of 178 and 138 W/m.degree. K respectively. The proportions of the metals in the composite are designed to adjust its overall coefficient of thermal expansion to be compatible with the material in the microcircuit component.
However, while copper, aluminum, and silver have specific gravities of less than 9, and melting points of less than 1,100.degree. C., tungsten and molybdenum have specific gravities of 19.3 and 10.2, and melting points of 3,370.degree. C. and 2,630.degree. C. respectively. Because of these large differences in the specific gravities and melting-points, and lack of mutual solubility of metals such as copper and tungsten, it is difficult to form composites of those two metals that exhibit a reliable degree of homogeneity using conventional melting processes.
One solution in which molten copper is infiltrated into a porous sintered compact of tungsten, as disclosed in Osada, et al. U.S. Pat. No. 5,086,333, consumes large amounts of energy and is therefore expensive. Another solution involves preclustering particles of both metals, then pressing and sintering the low melting point metal as disclosed in Polese et al., U.S. Pat. No. 5,413,751. However, it is difficult to predict or devise the optimum heating schedule for sintering the low melting point component in variously shaped and sized structures. Often times certain peripheral portions of the compact reach the sintering temperature before the deeper internal portions since heat is transferred from the outer portions to the inner. Prolonged exposure of the outer portion to these temperatures tend to reduce homogeneity.
Additionally, use of these dense metals increases the overall weight of the structures. In many applications such as in aerospace systems, lighter weight structures are preferable. Therefore, those materials having a high thermal conductivity and low density are desirable. In other words, for low-weight microcircuit applications, preferred microcircuit heat-dissipating materials will exhibit a high specific thermal conductivity, expressed by the ratio of its thermal conductivity coefficient and its specific gravity, or K divided by its density.
Although aluminum is light weight and has a high thermal conductivity, it has a very low melting point (about 660.degree. C.) and very high thermal expansivity (about 23.1.times.10.sup.-6 /.degree. C.). Prior composites of aluminum therefore suffer from expansivity problems. In addition, these composites are not readily brazable using relatively high-heat brazing techniques popular in the field such as CuSiL brazing which uses temperatures around 830.degree. C. Even short duration exposure to such temperatures will cause reflow of the aluminum out of the composite, reducing homogeneity and hence the composite's CTE compatibility. Also, the thermal conductivity of aluminum (about 237 W/m.degree. K) is significantly lower than that of copper (about 398 W/m.degree. K).
It would be advantageous therefore, to have a simpler and more practical process to manufacture homogeneous sintered composites using powder metallurgy techniques, and to devise a lighter weight, brazable, heat-dissipating material.
Porous metals have also been used as filtration media for separating solid contaminants from a liquid or gaseous stream. Metal filters enjoy several advantages over filters made from other media such as organic felt or paper depending on the application. Metal filters in general withstand higher temperatures and fluid volumes and pressures.
Porous metal filters have been used in many varying applications including petroleum refining, pharmaceutical, food, and beverage preparation, textile manufacturing and other chemical processing industries. In cryogenic applications, liquefied gasses such as oxygen, nitrogen, and helium are filtered. Porous stainless steel and MONEL brand nickel alloy metal have been used to filter molten uranium compounds in nuclear applications. Sintered bronze filters separate water from air in pneumatic systems via differences in surface tension.
In other filtration applications porous metals have been used as containment structures for other filtration media as in the membrane support in reverse osmosis filters and dialysis machines.
Porous metals are also useful for other applications apart from filtration such as in the fabrication of parts requiring transpirational cooling such as gas turbine and rocket engine parts, and even the nose tips of missiles. Porous metals have also been used to provide a base for tissue attachment in surgical implants.
The fabrication steps for creating the porous metal filtration media or other parts depends greatly on the application and the metals used. For example bronze filters are usually made by gravity sintering spherical bronze powders. The resulting properties of the filter depend on the particle size and porosity of the powder. Producing filters with the highest permeability for a given maximum pore size usually requires uniform size particles.
For stainless steel filter sheets, steel powder is mixed with resin, spread, then lightly pressed at a temperature that cures the resin. The sheet is then sintered decomposing the resin, then further densified by one or more pressings and/or sinterings. Porous hollow steel cylinders may also be formed by cold isostatic pressing.
In corrosive or high-heat applications other metals such as nickel-based alloys such as MONEL brand, INCONEL brand and HASTELLOY brand metal alloys are available in filter grade powders. The filtration media is then similarly formed using various costly and energy inefficient processing intensive metallurgical techniques.
In corrosive applications, refractory metals and their compounds such as titanium oxide or compounds such as silicon carbide are preferred as filter media due to their high corrosion resistance. However, most of these metals and compounds do not readily form alloys or homogeneous composites with other materials to result in an easily fabricated filter.
In high-heat applications, heat dissipation can be a problem. Therefore, in many processes where the filtrant is hot or in transpirational cooling, the porous metal material preferably has a high coefficient of thermal conductivity.
Another problem depending on the metals and fabrication techniques used is that many of the resultant filters can be exceedingly brittle. Further fabrication and machining to form complex shaped filtration media from such brittle stock is expensive and time consuming.
Lightweight filtration media are preferred for ease in handling, transport and in applications such as aerospace where low weight is preferred.
It would be advantageous therefore to have an economically produced lightweight material which is resistant to corrosives, has a porosity valuable for filtration, transpiration cooling, or tissue attachment and which is solid, machinable or otherwise readily formable into complex shapes and is heat-dissipating.